Complement receptors in neutrophils.

Human neutrophils contain numerous intracellular granules and vesicles that are exocytosed in a hierarchic manner on stimulation of the neutrophil with inflammatory mediators. Secretory vesicles are mobilized completely to the plasma membrane when neutrophils are stimulated with inflammatory mediators in nanomolar concentrations. There is evidence that secretory vesicles contain at least four different complement receptors, namely, C1qR, CR1, CR3, and CR4. These complement receptors are all transported to the neutrophil surface in parallel by the mobilization of secretory vesicles. A fifth complement receptor, C5aR, is constitutively present on the neutrophil plasma membrane. Because secretory vesicles are mobilized by virtually all inflammatory mediators known to stimulate neutrophils, a prompt upregulation of complement receptors is ensured early in neutrophil activation. The integrins CR3 and CR4 are located both in secretory vesicles and granules, and this diverse subcellular localization may reflect their functional versatility. The residence of complement receptors in several intracellular compartments ensures a graded upregulation of these receptors in response to stimulation.